(1) Technical Field of the Invention
The present invention relates to a positioning system for a vehicle.
(2) Description of the Prior Art
Heretofore, a positioning system used, for instance, for the route guidance unit for a vehicle employed for the most cases the system in which the vector that represents the shifted position of the vehicle is detected by means of a vehicle speed sensor and a direction sensor and the current position of the vehicle is found by integrating the values of the vector with respect to a reference position. In such a system, however, the detection errors in these sensors are accumulated as they appear so that it was necessary to provide some measures for removing these errors. With such a circumstance in mind, there has been proposed a positioning system which makes use of satellites called global positioning system (GPS) that is free from the problem of accumulation of these errors.
However, in GPS radio waves must be received simultaneously from three or more satellites to carry out a computation determining a current position of the vehicle, so that it fails to be feasible in an area where a plurality of GPS satellites cannot be observed.